The present invention relates generally to a motor control system for monitoring pulse width-modulated (PWM) control for a direct current (DC) electric motor, in particular for a fan motor for use in the motor vehicle sector.
German patent application DE 1 941 489 A1 has disclosed a protective circuit for a series circuit comprising a power semiconductor end stage and an inductive load, in parallel with which a freewheeling circuit comprising a freewheeling diode and a series-connected polarity reversal protection semiconductor switch is connected. The power semiconductor end stage can be switched on and off again as a function of a predetermined desired value with a pulse width-modulated control signal. The polarity reversal protection semiconductor switch can be controlled via a charge pump 36 such that it is not conductive when the polarity of the supply voltage has been reversed. As is further described in the mentioned laid-open specification, such protective circuits in the case of fault-free components provide sufficient polarity reversal protection, i.e. protection of the components against overload and destruction in the case of a supply voltage which is applied with the polarity reversed. However, if irregularities occur in such a circuit, uncontrolled overvoltages may occur in the circuit which can lead to destruction of components.
Accordingly, German patent application DE 199 41 489 A1 proposes developing the abovementioned protective circuit such that a point in the freewheeling circuit is monitored for a voltage which occurs when the freewheeling circuit is not functioning. Alternatively, it is proposed to monitor a point in the freewheeling circuit for a current which fails when the freewheeling circuit is not functioning. In the case of the occurrence of the fault voltage or in the case of failure of the current at the monitored point, the shutdown of the driving for the power semiconductor end stage will then be introduced. In particular, DE 199 41 489 A1 proposes monitoring the voltage drop across a field effect transistor (FET) arranged in the freewheeling circuit by means of a specially designed monitoring circuit. For this purpose, a buffer capacitor is connected in the freewheeling circuit at the connection point between the freewheeling diode and the polarity reversal protection semiconductor switch. The voltage at this buffer capacitor is then used as a monitoring voltage and is supplied to the motor electronics via a voltage divider.
One disadvantage with the protective circuit known from DE 199 41 489 A1 is the comparatively large number of cost-intensive components which are required for the protective circuit. Furthermore, the protective circuit requires continuous operation of the driven electric motor during pulse width-modulated operation in order for it to function properly. Operation at a duty cycle of 100% is not possible. This results in disadvantages in terms of the efficiency of the motor control system.